This invention pertains to an apparatus for accelerating the weathering effects on test specimens, and more particularly, to an accelerated weathering apparatus having discharge lamps as a concentrated light source for accelerating the deterioration of color, composition, and/or structure of test specimens which includes improved control and calibration structure and methods of operation. The preferred embodiment of the present invention for testing specimens uses fluorescent ultraviolet lamps as the light sources that approximate natural sunlight in the ultraviolet portion of the spectrum and the deterioration caused thereby and will be described with particular reference thereto. However, it will be recognized that other discharge lamps may be used as sources, the primary example being xenon lamps or any other source.
A conventional testing apparatus using discharge lamps as shown in FIG. 1 has eight ultraviolet lamps 10 provided in a test chamber 12 and arranged into symmetric downwardly divergent rows when viewed in cross-section. Specimens 14 to be tested are attached to two opposite specimen supporting walls of the housing of the test apparatus so as to face inwardly toward the lamps and receive the irradiance therefrom. In the machine shown, there are two specimens, an upper specimen and lower specimen; however, there may be only a single specimen or more than two. The rear surface of the specimens 14 is exposed to the atmospheric air outside the machine. Outside air is heated and blown into the interior of the chamber 12 to regulate the temperature in the chamber 12. Water in the moisture supply tank 16 is heated by conventional means and evaporated to supply moisture into the chamber 12.
In the above-described testing machine, one example of the machine""s operation includes applying irradiance rays to the specimens 14 at a temperature of 60xc2x0 C. for sixteen hours and then the lamps 10 are turned off and the interior of the chamber 12 is kept at 50xc2x0 C. to create humidity for eight hours. These two steps, which constitute one cycle of a deterioration testing operation, are repeated continuously. While the lamps are off, the humidity in the chamber 12 is high, and the rear surface of the specimens is exposed to the outside air at a low temperature. Accordingly, the surfaces of the specimens are wetted due to condensation. Thus, the wetting of the specimens, the applying of ultraviolet irradiance, and the drying are repeated, which speeds the deterioration of the specimens. It is to be appreciated that the above description is just one type of cycle for which machines of this nature can be used.
Problems, however, exist with the apparatus shown in FIG. 1. Initially, there is no provision for sensing the output of the fluorescent lamps 10, in order to track their rate of degradation or control the irradiance output. A normal procedure for attempting to provide a uniform output from the lamps, in such a device, is to rotate the positions of the lamps at predetermined time intervals in a predetermined sequence. Testing of the lamps to detect actual output is not provided; rather, assumptions are made as to the likely output, and the rotation sequence is made in consideration of the assumptions.
Another drawback of this type of device is that there is no provision for conditioning the lamps during start-up and operation. Accordingly, the life of the lamps is compromised and the accuracy of any test is skewed. There is also no ability to calibrate the irradiance emitted from the lamps.
Various attempts have been made to improve on the above-noted drawbacks of the conventional testing apparatus shown in FIG. 1. Among these is an apparatus from Atlas Electric Devices Company, called Atlas Ci35 FADE-OMETER(copyright); an apparatus from Heraeus called XENOTEST(copyright) 1200 CPS; U.S. Patent to Suga, U.S. Pat. No. 4,544,995 issued Oct. 1, 1985; U.S. Patent to Kockott, et al., U.S. Pat. No. 4,544,995 issued Apr. 27, 1971; and U.S. Patent to Fedor et al., U.S. Pat. No. 5,206,518 issued Apr. 27, 1993.
The Atlas device is arranged for use with a xenon arc lamp and includes a closed loop irradiance monitor as its primary light control system. The monitor, using a light pipe, interference filter and photosensitive diode feeding into solid-state electronics, maintains predetermined irradiance levels and totalizes the energy received by the samples through an integrator. This device is also equipped with manual irradiance controls for use when periodically calibrating the system.
The apparatus from Heraeus is also directed for use with xenon arc lamps. This device employs three light detectors to detect the output of three individual xenon arc lamps.
A conventional apparatus including elements of these two above-discussed devices includes discharge lamps, which can be of a xenon type, that are vertically disposed. A filter surrounding the discharge lamps is provided to allow only desired wavelengths of light to pass. Sensors are provided to sense the output of the vertically positioned discharge lamps, and a rotating specimen holding rack is positioned to encircle the discharge lamps. Each of the detectors is provided to detect the irradiance produced from a respective discharge lamp over time. The rotating specimen holding rack rotates the specimens located in the specimen holding rack. The sensors are provided to track the output of the discharge lamps, and the rotating specimen holding rack attempts to provide each of the specimens with an average overall equal amount of irradiance. Inner walls are used to direct reflective light of the discharge lamps outward to the specimens.
Another device, employing ultraviolet lamps in an arrangement similar to FIG. 1, is known to include a single sensor. However, in such an arrangement it is necessary to match the characteristics of the lamps prior to placing them in such a device. This is required since the sensor will sense only the lamps closest to its location. Thus, the sensor will assume the lamps placed distant from it are operating the same as the lamps it actually senses.
The Suga patent attempted to improve on the prior art device shown in FIG. 1 by adjusting the alignment of the row of discharge lamps 10 of FIG. 1 into a nonsymmetric arrangement. The discharge lamps 10 are not disposed immediately below each other. Rather, they are in a specifically positioned arrangement. This was done in Suga in an attempt to provide irradiance to the samples 14 with a more uniform distribution.
The Kockott, et al. patent is directed to a device using an elongated source of irradiation inside a cylindrical carrier surface. Kockott, et al. discloses three approaches to provide a uniform distribution of irradiance to the samples. First, mirrors are arranged to reflect usable light; second, a light source is designed to increase light intensity at its ends; and, third, collimating discs are used to inhibit divergence of the radiation emitted from the source.
The Fedor et al. patent is directed to an apparatus similar in structure to the apparatus shown and described in FIG. 1 which has an improved light output controller and light beam distribution in the test chamber. Fedor et al. discloses an apparatus including a housing with a test chamber and a specimen supporting wall located inside of the chamber. A light source is provided in the test chamber. A ballast arrangement is connected to the light source for controlling the amount of power the light source receives from a power source. A controller is connected to the ballast arrangement, to produce a ballast control signal for controlling operation of the ballast arrangement according to a desired set-point value. A light source detector is disposed within the specimen supporting wall in order to detect irradiance existing in the test chamber so the light source detector can generate an irradiance signal, which is then input to the controller. The controller uses the irradiance signal to adjust the ballast control signal to maintain the selected set-point value. A calibration portion includes a reference detector inserted into the specimen supporting wall adjacent the light source detector, which is designed to detect the irradiance inside the test chamber and to produce a reference irradiance signal. The reference irradiance signal is transmitted to a calibration meter, which produces a calibration signal. The calibration signal is transmitted to the controller for calibrating the apparatus.
The Fedor apparatus also includes a barrier wall located within the test chamber. The barrier wall is configured to selectively block and divert beams of light produced by the arrays of light sources. The blocking and diversion of the beams occur in a pattern selected to increase an even distribution of the beams to the specimen supporting wall.
The Fedor apparatus still further includes a plurality of concurrently-operating, automatically-adjusted control channels for controlling output of the individual light sources. The channels control the output of at least one of the light sources.
While the above-discussed references provide some improvements upon the conventional apparatus discussed above, drawbacks still exist.
With particular attention to the Atlas and Heraeus devices, it is noted that both use a rotating specimen rack arrangement. This rack is necessary for a very basic reason. The Atlas device includes a monitoring system that monitors the overall output of the xenon arc lamp in order to attempt to maintain a predetermined total irradiance output level over time for the entire system. The Heraeus device uses three sensors to control the three different lamp""s output over time. These sensor arrangements are used to produce an irradiance that is constant over time. However, neither of these devices use a sensing arrangement to make irradiance constant over space.
Both of the devices use a rotating specimen rack in an attempt to achieve spatial uniformity. Therefore, spatial uniformity is accomplished by having the specimens in the rotating rack revolve around the lamps, so the effective light dosage received by each specimen is an average of the different irradiances at each point on the circumference of the sample plane. Though rotating the rack increases uniformity, it also increases the complexity of the device by requiring a motor and associated rotation mechanisms.
Thus, even though these devices include irradiance sensing capabilities, they implement these capabilities only for a consistent output over time, not space. Due to the geometry of the devices, there is a different irradiance at every point around the circumference of the sample plane. Therefore, areas that are located in front of a discharge lamp will have a high irradiance area while samples that are at a position distant from a discharge lamp will receive lower irradiance. Rotation of the rack attempts to produce an overall average uniformity of irradiance impinging upon samples.
The known ultraviolet system using a single sensor includes the drawback of needing to match the lamps being used in the system. This requires extensive testing of the lamps prior to use. A further drawback is that in such a system, when a lamp located distant from the sensor location burns out or degrades, the decrease in its output will not be sensed. This is true as only the nearest lamps are actually sensed and an assumption is made that the remaining lamps are functioning in a similar manner.
The Suga patent attempts to increase the uniformity of light impinging upon specimens by moving the center two lamps away from the samples to increase uniformity of light to the samples from top to bottom. A drawback of such an arrangement is that it is not possible to easily retrofit existing weathering devices to gain whatever improvement there may be from the Suga arrangement.
A drawback to the Kockott, et al. patent is that it is directed to single lamp systems. Another drawback to Kockott, et al. is that it increases the complexity and cost of the apparatus.
A further drawback associated with the conventional testing apparatuses as discussed above is their calibration. These devices require manual manipulations by an operator, which in turn means the operator is required to make decisions that are critical to proper calibration. Since the operator is responsible for making decisions while manually re-calibrating the apparatus, the accuracy of the calibration will be dependent upon the skill of the operator. Additionally, since the calibration is accomplished manually, extended down time occurs during such calibration and there exists a substantial possibility of inaccuracies due to operator error.
The Fedor patent attempts to automate the control and calibration procedures for the testing apparatus. The control concurrently monitors a plurality of sensors disposed within the specimen supporting wall and controls each separate channel individually. To calibrate the individual sensors, the operator opens the door and installs a reference sensor within the specimen supporting wall inside the testing chamber immediately adjacent the individual sensor. The calibration procedure unfortunately introduces a great deal of operator error. The operator must manually select the type of lamp and the location for each calibration position. A disadvantage of this calibration procedure is that the operator must bypass the safety system which further introduces error into the reference sensor readings. Furthermore, the operator is exposed to harmful ultraviolet radiation.
Fedor also attempts to control the distribution of the light beams within the chamber by placing a barrier wall between the arrays of lamps. A drawback of such an arrangement is that the individual sensors and reference sensor are installed within the specimen supporting wall. In this location, the sensors are exposed to all of the weathering effects that are witnessed by the test specimens. Accordingly, the sensors deteriorate when used with this apparatus, thereby introducing error into the tests. Another drawback is the ballast used to control power to the lamps. Fedor discloses the use of only conventional ballasts, in which a signal is sent to the ballast to increase or decrease power from the ballast.
A still further drawback of the Fedor apparatus is the use of a concurrent control algorithm, which results in a biased sensor reading. As a result, the measured value of irradiance is inaccurate. Therefore, the control system has an error bias and the test results cannot be trusted as accurate.
Therefore, there is a need for an improved accelerated weathering apparatus that has an improved sensor location, unbiased sensor readings, improved control and calibration methodology and improved ballast construction and operation.
The subject invention contemplates a new and improved accelerated weathering apparatus that overcomes all of the above referenced problems and others and provides an easily operated, reliable testing structure.